Saturday, March 12, 2016

The mainstream dinosaur-to-bird thinking is that Paraves is a node on a
proposed lineage from dinosaur to bird. The thinking is that basal Paraves was not
a power flier. However, the evidence shows that basal Paraves was a power-flying,
primitive bird. It had extensive flying bird-like characteristics.

1.0 Materials and methods

An extensive review and analysis was done of the literature
concerning the basal Paraves. The mainstream idea is that basal Paraves were
non-flying. However the evidence supports the idea that basal Paraves
were powered fliers.

To begin, let us look at the characteristics of the basal
Paraves as seen in the published material:

It has been hypothesized that bird flight went through a
four-winged “tetrapteryx" stage.
Epidendrosaurus, Epidexipteryx and
Eosinopteryx, are basal paravians. Their phylogeny is entirely
consistent with the presence of a tetrapterygian condition (= four
winged) and elongated rectrices in basal Paraves. (Godefroit et al, 2013a)

Large pennaceous feathers are now
known to occur on the lower leg and particularly the metatarsus of at least one
basal member of each of the three major paravian groups, namely the basal
troodontid Anchiornis, the basal avialan Pedopenna and the basal
dromaeosaurid Microraptor. This led to a four-winged conditionat the base of the
Paraves. (Hu et al2009)

1.3
Arboreal

Scansoriopterygidae
(a basal Paraves) had clear adaptations to an arboreal or
semi-arboreallifestyle–it is likely that they spent much of their time
in trees.

Scansoriopteryx is considered to be arboreal based
on the elongated nature of the hand and specializations of the foot. The long
hand and strongly curved claws were adaptations for climbing and moving around among tree branches. (Zhang et al.2002).

Paraves were members of Pennaraptora and thus had this
characteristic.

1.5
Long and robust forelimbs

The significant lengthening and thickening of the
forelimbs indicates a dramatic shift in forelimb function atthe base of the Paraves,
which might be related to the appearance of a degree of aerodynamic capability
(Xu et al al2011)

The evolution of enlarged
forelimbs is strongly linked, via whole-body centre of mass, to
hindlimb function during terrestrial locomotion. The evolution of avian
flight is linked to anatomical novelties in the pelvic limb as well as the pectoral. (Allen et al 2013)

1.8 Brain

If
Archaeopteryx has a ‘flight-ready’ brain, which is almost certainly the
case given its postcranial morphology, then so did other paravians. The
hypothesis that dromaeosaurs and troodontids had the neurological capabilities
required of powered flight,
gliding, or some intermediate condition is congruent with the discovery of the
‘four-winged’ deinonychosaurs, Microraptor zhaoianus and Anchiornis huxleyi (Balanoff et al2013)

1.9 Uncinate processes

The uncinate processes in non-avian
maniraptoran dinosaurs [paravians] are not reduced as in running birds but resemble those of the flying or diving
birds. (Codd et al2008)

We have
seen the extensive set of flying bird-like characteristics they had. Could they
fly?

We need
to look at the characteristics related toforewings, hindwings,
airfoil, sternum, propatagium, semilunate carpal and feathers. And we need
to consider possible objections.

2.1. Forewings (Shoulder mechanism)

It is sometimes thought that the
primitive birds could not fly because they did not have the needed shoulder
mechanism for powered flight. It is true that the lack of a derived
supracoracoideus precluded takeoff from the ground. But takeoff from an
elevated perch would still be possible.

The dorsal
elevators, principally the deltoideus major, can effect the recovery stroke by
themselves, as they did in Archaeopteryx. Maxheinz Sy proved this when he
cut the tendons of the supracoracoideus in living crows and pigeons (1936). Sy found
that pigeons were capable of normal, sustained flight; the only
capacity they lost was the ability to take off from level ground.(Feduccia1999)

Microraptor lacked the necessary adaptations in its
shoulder joint to lift its front wings high enough vertically to generate lift
from the ground. This leaves only the possibility of launching from an
elevated perch and even modern birds do not need to use excess
power when launching from trees, but use the downward-swooping technique
found in Microraptor. (Chatterjee, Templin2007).

2.2. Hindwings

It is sometimes thought that the
primitive birds lacked sufficient lift. But the hindwings generated additional
lift.

In
Microraptor the metatarsal feathers are similar in general arrangement (nearly
perpendicular to the metatarsus, forming a large flat surface) and in having
stiff vanes and curved rachises. These features suggest that the metatarsal
feathers were aerodynamic in function, providing lift and
thus played a role in flight (Zheng et al 2013)

Microraptor
gui preserves evidence of extensive, lift-generating feathers on each manus and
forearm, but also preserves evidence of lift-generating feathersassociated
with the hindlimbs, effectively forming a pair
of “hindwings”. ( Hall et al 2012)

2.3. Airfoil

It is
sometimes thought that the primitive birds could not fly because they did not
have asymmetric flight feathers and
thus lacked an airfoil. It is true that they did not have asymmetric flight
feathers. But that does not preclude powered flapping flight.

Although
the slender feather shafts of Archaeopteryx and Anchiornis make individual feathers weak, layering
of the wing feathers may have produced a strong airfoil. (Longrich et al 2012)

The elongated wing feathers of primitive birds
exhibit small barb angles in cutting-edge leading vanes that are comparable
with those of modern flying birds. This suggests that the leading
vanes of these Mesozoic feathers were similarly capable of withstanding
aerodynamic forces in airflow. (Feo
et al2015)

2.4. Sternum

It is sometimes thought that the
primitive birds could not fly because some did not have an ossified sternum. It
is true that some did not have an ossified sternum but that does not preclude
flapping flight.

An ossified sternum and uncinate processes
are absent as in Anchiornis, Xiaotingia and troodontids. (Xu et al2011)

But:

Thesupracoracoideus muscle, and hence an ossified
sternum, is not necessary to effect the recovery stroke of the wing. There is nothing in the structure of the pectoral
girdle of Archaeopteryx that would preclude its having been a powered
flier. (Olson, Feduccia, 1979)

Ossified
sternal plates are known in basal
dromaeosaurids, oviraptorosaurs, and scansoriopterygids, forming a fully fused
sternum in some individuals of the first two groups. (O'Connor, Sullivan 2014)

2.5. Propatagium (Lift)

The
lift generating effect of the propatagium must also be considered.

The cambered propatagium is the
major lift generating component of the wing proximal to the wrist. (Brown et al 1996)

2.6. Semilunate carpal

Paravians
are characterized by long arms and three-fingered hands as well as a
"half-moon shaped" (semi-lunate)
bone in the wrist (carpus).

Scansoriopterygids had a semilunate carpal (half-moon shaped
wrist bone) that allowed for bird-like folding motion in the
hand. By folding its wings (decreasing the wingspan) a bird can reduce
drag during the upstroke.

Anchiornis
exhibits some wrist features indicative of high mobility,
presaging the wing-folding mechanisms seen in more derived birds and suggesting
rapid evolution of the carpus. (Xu et al2009)

2.7. Weak Feathers

It is
sometimes thought that the primitive birds could not fly because their feathers
were too weak. However:

The relatively weak-looking flight feathers of basal birds,
do not necessarily suggest that flight capability was poor, let alone entirely
absent. Early birds and their close relatives could assemble an
effective flying wing using multiple rows of relatively weak feathers.
(Longrich et al2012)

2.8 Uncinate processes

It is sometimes thought that the primitive birds could not
fly because some lacked uncinate processes.

Thescreamersare a smallcladeofbirds(Anhimidae)The clade is exceptional within the
living birds in lackinguncinate
processes of ribs.[3] (Fowler ME & Cubas ZS
(2001).Biology,
medicine, and surgery of South American wild animals. Wiley-Blackwell.
p. 103.)

Some basal Paravians had uncinate processes:

The uncinate processes in non-avian
maniraptoran dinosaurs [paravians] are not reduced as in running birds but resemble those of the flying or diving
birds. (Codd et al2008)

2.9 Apomorphy of Avialae

Also, we can conclude that basal Paraves were capable of
flapping flight because their flight-related characteristics have often caused
them to be placed within Avialae:

Avialae is defined as an apomorphy-based clade that“possessed
feathered wings used in flapping flight, and the birds that descended from them.”

For example, Pedopenna was originally classified as a paravian, but some scientists have classified it as
a true avialan.

3.0 Problems with the
idea that basal Paraves were ground dwellingThere are a number of problems with the mainstream idea that basal Paraves were
ground dwelling (non-flying).

Problems include:

·flight had to evolve
multiple times independently (homoplasy).

·it requires numerous
exaptations (they were not using their bird-like characteristics for flying)

·the hindwing
feathers would interfere with running

3.1 Extensive Homoplasies
RequiredConvergent evolution createsanalogousstructures that have similar form or
function, but that were not present in the last common ancestor of those groups.The cladistic term for the same
phenomenon ishomoplasy.

Apolyphyleticgroup is characterized by one or
more homoplasies.

Many biologists aim to avoid homoplasies in
grouping species together and therefore it is frequently a goal to
eliminate groups that are found to be polyphyletic. This is often the stimulus
for major revisions of the classification schemes.

As a result of the high amount of
homoplasy that characterizes derived
maniraptoran evolution, the identity of the avian sister taxon remains debated
despite the rapid accumulation of morphological data. (O’Connor, Sullivan2014)

Flight capability is likely to
have evolved independently on multiple occasions among
Archaeopteryx and its kin (Longrich et al2012)

Xiaotingia zhengi independently evolved some salient features
seen in other maniraptoran taxa, which highlights the extensive
homoplasy that exists among maniraptorans.( Xu et al 2011)

Thus, bird-likeencephalization
indices evolved multiple times, supporting the
conclusion that if Archaeopteryx had the neurological capabilities required of
flight, so did at least some other non-avian maniraptorans. (Balanoff et al2013)

3.2
Numerous Exaptations Required

Exaptation and
the related term co-option describe a shift in the function of a trait
during evolution. For example, a trait can evolve because it served one
particular function, but subsequently it may come to serve another.

In the dinosaur to bird theory, there are an extensive
number of required exaptations.

Abducted wrists,
feathers, enlarged brains and laterally oriented, long and robust
forelimbs and forelimb myology and
breathing apparatusare claimed to have evolved before they were used for
flight.

It is likely that mobility of the wrist was initially
associated with other functions, such as predation. Partial folding of the wing during the
upstroke in extant birds, which requires significantabduction of the wristcould then be seen as an exaptation. (Sullivan
et al2010)

Feathers

Researchers have speculated early feathers may have
been used for attracting mates or keeping warm. But later on, feathers became essential for modern
birds’ flight.

This further supports the hypothesis that "flight
feathers" that first evolved for non-aerodynamic functions were
later exapted to form lifting surfaces. (Dyke et al2013)

Enlarged
brains

If
Archaeopteryx had a ‘flight-ready’ brain , which is almost certainly the
case given its postcranial morphology, then so did other paravians. Paraves had
the neurological capabilities required of powered flight, gliding, or some intermediate condition.(Balanoff et al 2013)

Laterally
oriented, long and robust forelimbs

Basal paravians had many hallmark
features necessary for flight, including a laterally oriented, long, and robust
forelimb. ( Xu et al 2014)

Forelimb myology and
breathing apparatus

The origin and evolution of flight were more complex than
previously thought, and forelimb myology and breathing apparatus could draw on structures that evolved in different
functional contexts. [exaptation] (Foth et al2014)

Attributed to Pennaraptora:

advanced costosternal ventilator pump (Xu et al2014)

Objection

Current exaptational explanations are often not fully
formulated and rarely offer a biologically plausible hypothesis to
account for their origin. (James,
Pourtless2009)

In both Anchiornis and
Microraptor the long metatarsal feathers would have interfered with terrestrial
locomotion（Xu
et al., 2003, Hu et al., 2009）(Sorkin2014)

4.0 Possible Objection

It may be objected
that exaptations must have occurred, because the flying bird-like
characteristics were around before basal
Paraves.

The counter to this
is that many of the flying bird like characteristics appeared for the first
time at the base of the Paraves.

The evidence indicates that the Paraves flying
bird-like characteristics appeared at the base of Paraves.

Before the origin of Aves, on the branch leading to Paraves, high rates of
evolution led to a smaller body size and a relatively larger forelimb in
Paraves. These changes are on a single branch leading to Paraves, representing
a shift to a new smaller size and larger forelimb at this point.Paraves, rather than Aves alone, shifted to a different evolutionary
model. On all trees and for both femur and forelimb size, the model with
a regime shift at Paraves, rather than Aves, is
favored. (Puttick et al 2014)

This suggests that large pennaceous feathers
first evolved distally on the hindlimbs, as on the forelimbs and tail. This
distal-first development led to a four-winged condition at the base of
the Paraves. (Hu et al 2009)

The significant lengthening and thickening of
the forelimbs indicates a dramatic shift in forelimb function at the base
of the Paraves, which might be related to the appearance of a degree of
aerodynamic capability (Xu et al 2011)

Paraves, exclusive of Epidexipteryx hui, is marked by a suite of
modifications to the shoulder girdle typically associated with the origin of
the ‘‘avian’’ flight stroke (Ostrom, 1976b; Jenkins, 1993). The acromion
margin of the scapula has a laterally everted anterior edge (char. 133.1) (fig.
55), the coracoid is inflected medially from the scapula forming an
L-shaped scapulocoracoid in lateral view (char. 137.1) and the glenoid
fossa faces laterally (char. 138.1) as opposed to the plesiomorphic
posterior orientation (fig. 50). Additionally, the furcula is nearly
symmetrical in shape as opposed to the asymmetry present in the furcula of more
basal taxa (char. 474.1).
(Turner et al. 2012)

For example, the early evolution of paravian theropods
features cerebral expansion and elaboration of visually associated brain
regions (71), forelimb enlargement (22, 67), acquisition of a crouched,
knee-based hindlimb locomotor system (67), and complex pinnate feathers
associated with increased melanosome diversity, which implies a key
physiological shift (72). Together these features may suggest
the appearance of flight capability at the base of the Paraves (22, 67). (Xu et al 2014)

5.0 Conclusion

Altogether the evidence is substantive that Paraves was a power-flying,
primitive bird. This hypothesis is supported by the evidence and does not
require the extensive amount of exaptation and convergence that the current
mainstream hypothesis requires. It is thus the more plausible, parsimonious
hypothesis.

The next question is: What was theancestorof flying Paraves like? That is the
direction future research could take.

Pp. 7-41 in Gauthier, J. and L.F. Gall
(eds.),New
Perspectives on the Origin and Early Evolution of Birds: Proceedings of the
International Symposium in Honor of John H. Ostrom. New
Haven: PeabodyMuseum
of Natural History, YaleUniversity.

Note

This site presents the idea that pterosaurs (rather than dinosaurs) developed into birds. This is not an "evolutionism" vs. "creationism" issue.An "evolutionist" can say that the pterosaur to bird developments are due to neo-Darwinian means (random mutation and natural selection).On the other hand, a "creationist" can say that those developments are the acts of a higher intelligence.This site does not take a position on the "evolutionism" vs. "creationism" question.

Philosophy

Like most people, I have philosophical ideas that go beyond the nuts and bolts of the scientific analysis of the origin and development of birds.There are larger questions that philosophers have grappled with since the most ancient times. If anyone is interested in my take on those more philosophical ideas, click here.But please realize that all the ideas of this site are pure materialist, scientific ideas supported by physical evidence and scientific studies.

The 19th-century German philosopher Arthur Schopenhauer astutely summarized the three stages through which all truth passes: first, it is ridiculed; second, it is violently opposed;

and third, it is accepted as being self-evident.

"In the choice between changing one's mind and proving there's no need to do so, most people get busy on the proof."~John Kenneth Galbraith

Keywords

origin of birds, pterosaur is the ancestor of modern birds, birds did not evolve from dinosaurs, cladistics, stratocladistics, Cretaceous, Mesozoic, fossil record, BAD, BAND, birds are not dinosaurs, flightless birds, aves